High temperature gas confinement arrangement



Nov. 10, 1964 v. JOSEPHSON r 2 HIGH TEMPERATURE GAS CONFINEMENT ARRANGEMENT Filed D60. 14, 1960 CIRCUIT SWITCH CONTROL 222? 5 F I G. l. GENERATOR GAS INLET PULSE LINE I I SWITCH J 30 7 52 .4 lcmcurr H R.F. ELECTRIC lo FIELD VECTORS CATCHER CAVITY 4| 50 RF. MAGNETIC FIELD. I2 j 20 2s AMPLIFIER E CAVITY g c agAem-mc I e 22 F I G. 3.

2| 1 BUNCHER I CAVITY RF I9 20 l INPUT VERNAL JOSEPHSON INVENTOR.

32; BY @M'JFW CATHODE PULSE SOURCE F I ATTORNEY Calih, assignor Los Angeles,

The present invention generally relates to the magnetohydrodynamics art and more particularly to apparatus United States Patent for the generation and confinement of high temperature gases.

It is a well established fact that high temperature gases are useful for investigation or to initiate a thermonuclear reaction, or to provide neutrons. Since the temperatures involved in thermonuclear processes are of the order of 10' degrees Kelvin, conventional heating techniques are not suitable for bringing the gases to the desired high temperature or energy levels.

It has been found possible to obtain the desired high energy levels by accelerating particles of a gaseous medium to high kinetic energies and then containing them or, stated dilierently, confining them to some small volume. In one technique, anionized gas (a plasma) is confined at a low pressure within a selected volume and a high density current is then driven through the plasma. The high density current establishes an external D.C. magnetic field which constricts the plasma (drives the plasma particles inward) toward the axis of current path. The phenomenon by which magnetic fields function to constrict the gas is usually referred to as the pinch effect.

After the initial establishment of the pinch effect, a difficult problem generally arises in the stable containment of the pinched plasma. Various instabilities in the plasma itself and in the containment apparatus and technique have usually limited the existence of the pinch effect to extremely short time periods of the order of microseconds or less. Devices which utilize a radiofrequency magnetic field in addition to the D.C. magnetic field may be arranged to extend the duration of the constriction. They should not, of course, have an arrangerncnt which will interfere with the structure needed to initiate the pinch effect, but should permit etficient coupling of high radio-frequency power levels directly to the plasma. Moreover, the radio-frequency fields should be generated by a very low impedance arrangement which is not subject to high R.F. electric field break down in the region of extremely low impedance plasma generating structures. Recent developments indicate that it is desirable 'to have the pinched plasma inside the R.F.

cavity, which construction also provides simple access to the D.C. driving arrangement which can establish the pinch effect.

- It is, therefore, an object of this invention to provide new and improved apparatus for the stable containment of a plasma by the combination of direct-current and radio-frequency magnetic fields.

The present invention overcomes the above and other disadvantages and limitations encountered in prior art apparatus by combining the structure of the plasma-containing chamber and the resonant cavity of a microwave power generator. According to the present invention, the chamber is made a part of a resonant cavity design so that all of the necessary electromagnetic fields are established simultaneously in the cavity and in the chamber.

More particularly, in one form of the invention, a high power klystron has a resonant catcher cavity concentric with a plasma-containing toms. A D.C. magnetic drive on the outer surface of the torus is energized to initiate 3,156,621 Patented Nov. 10, 1964 'ice will be seen from the following detailed description, the

windings of the D.C. magnetic field are arranged outside of the torus and therefore will not interfere with the RF. fields.

The subject matter which is regarded as this invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, as to its organization and operation, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective sectional view, partly in schematic form, of one klystron cavity arranged in accor ance with the present invention;

FIG. 2 is a schematic view of a multi-cavity power klystron, useful in the present invention; and

FIG. 3 is a simplified representation of a portion of the electric field distributions of the arrangement illustrated in FIGS. 1 and 2.

Referring now to the drawing wherein like numbers refer to similar parts, a high power microwave signal generator is provided with a resonant cavity 10 (FIGS. 1 and 2) of a multi-cavity power amplifier klystron (FIG. 2). The cavity 10 also defines a plasma-containment region 12 therein. Circuits associated with the plasma-containment region 12 in FIG. 1 for initiating heating of the plasma by means of a dense pinched current include a pinched field generator 14 and a short circuiting device 16. In FIG. 2 the dense pinched current is provided by a low impedance long pulse unit or delay line 15. The pinched currents are provided in such a way that the microwave fields are properly disposed to assist in containing the pinched plasma.

Specifically referring to FIG. 2, a preferred form of the multi-cavity power amplifier consists of a klystron tube 17- concentrically disposed about a central axis 18. The klystron has an electron discharge section along the central axis which cooperates with the klystron resonant cavity 10 to provide a velocity modulated or bunched stream of electrons. At what may be called the electron gun end of a klystron tube 17, a large electron source or cathode 19 is arranged to be energized with a large negative pulse relative to a buncher cavity 20 positioned along the central axis 18. In accordance with conventional multi cavlty klystron design, the buncher cavity 20 is energized with a low power R.F. signal applied through a hne 21 to develop a high frequency signal during current flow through the klystron tube 17. The low power RF. signlal tapplied to the buncher cavity 20 causes successive eec rons to travel at slightl dilferent ve c' whereby they will be bunched duriri g passage alci g i liz central arms 18 in the region of an amplifier cavity 22 The flow of bunched electrons through the region of the amplifier cavity 22 causes R.F. oscillation'therein which Is effective to further vary the velocity of the electrons to substantially increase the effective bunching. As these bunched electrons pass through the region of the c t h cavity 10, they impart thereto a substantial portion of their energy.

Because of they tend to sz fte i r t iif g i l ii the bunc'hed electrons s uring then traversal of 3 the klystron tube 17. In order to maintain focusing of the electrons, the concentric focusing coils 24 and 25 are positioned along the klystron tube as indicated. The

- ing. The RF. signal frequency chosen must match the resonant frequency of the buncher and catcher cavities in order for maximum amplification. In the present device the klystron oscillates at a frequency selected accord ing to the size and configuration of the resonant cavity 10.

Referring now more specifically to FIG. 1, the resonant catcher cavity 10 of the klystron 17 is a hollow toroidal member concentric with the central axis 18. The resonant cavity 10 is completed by a pair of parallel radially extending plates 36 37 which are afiixed to the tube envelope to provide electromagnetic coupling therewith. The parallel plates 36, 37 define the capacitance region of the resonant cavity 10.

In order to establish the pinch effect in conjunction with the microwave generator, the re-entrant cavity 10 is provided with a plurality of radial grooves 40. These grooves 40 electrically isolate tangentially adjacent segments of the resonant cavity 10 and thus prevent shunt ing of the dense pinch currents. A hollow torus 41 of insulation material such as a ceramic is mounted concentrically with the segmented toroidal resonant cavity 10. The axis of revolution of the torus 41 coincides with the central axis 18 of the klystron tube 17.

A preferred method for mechanically combining the elements of the resonant cavity 10 and the torus 41 is to fabricate the ceramic segments of the torus 41 in two pieces having a line of mating surfaces along the plane defined by the clamping flanges 42. The inner surfaces of portions of the torus 41 thus formed are then coated with a conductive layer which will subsequently form the segmented resonant cavity 10. The segmentation is accomplished by cutting through the conductive layer at a plurality of tangentially spaced locations as indicated by the grooves 40. After these portions are formed, they are clamped together at the flanges 42 and also clamped in the region of gas-tight seals 44 whereby the plasma containment region 12 will maintain a vapor pressure of deuterium of about 100 microns. The clamping arrangements are preferably relatively simple and may consist of a plurality of bolts 45 in the region of the flanges 42 and a pair of nuts 46 in the region of the seals 44. The nuts 46 may be threaded directly on the klystron tube 17 or may be threaded on collars 47 secured thereto. It is requisite to the present invention that the gas-tight seals 44 develop low impedance, electric couplings between the resonant cavity 10 and the radial extending plates 36 and 37. In order that the vacuum within the klystron may be maintained, it is a current practice to market such tubes with a ceramic sleeve 48 secured thereto as an integral unit. It is on this region of the klystron 17 that the torus 41 is mounted.

What may be called a linear pinch drive is provided about the torus 41 by linear, as distinguished from helical, windings 50 about the torus 41. That is, each of the windings 50 is coaxial with the axis of revolution (18) of the torus 41. Each of the windings 50 is connected to the leads, as illustrated by the numerals 52, which are selectively energized by the pinched field generator 14. It is preferred that the leads 52 be separated by dielectric material such as a radial flange 53 which may be an integral part of the torus 41. The pinched field generator 14 is basically a high energy current source exempified by a large current storage capacitor. The short circuiting device 16 is selectively coupled in parallel to the pinched field generator 14 through a switch control 55, one set of switches being illustrated at 56. The short circuiting device 16 operates at the proper time (during maximum pinch current flow) to provide a closed current loop for the linear windings 50. The proper time is selected by the switch control 55 which may be a time delay device such as a relay or a delay line.

In operation, the arrangement of FIG. 1 first generates a pinch effect in the plasma 51 and then provides stable containment of the pinchedplasma through the operation of the klystron tube 17. While there is an integrated action between the various portions of the apparatus in providing the over-all result, the operation of the pinch effect may conveniently be considered first.

To initiate operation, the pinched field'generator 14 passes a current through the linear windings 50, which current very rapidly rises to a large magnitude. The result is a sharp rise in the current flow about the torus 41 to generate a circular magnetic field which extends into the region of the plasma 51. Counteracting currents are induced in the plasma 51 rather than in the resonant cavity 10 because of the isolating properties of the grooves 40. Maximum currents developed in the plasma 51 cause the magnetic field and the necessary pinch efiect. This magnetic field encircles the path of the current carrying plasma 51 and accelerates the particles of the plasma 51 inwardly.

When the current provided in the linear windings 50 has reached a maximum value, the switch control 55 closes the switches, including the set of switches 56, whereby the short circuiting device 16 operates elfectively to shunt the current of linear windings 50 around-the torus 41. This technique of closing the current path to maintain current fiow at a maximum level has acquired the name crowbarring and has the objective of maintaining the maximum current level for as long as is practicable. As illustrated in FIG. 2, the switch control 55 may also be used to connect the low impedance, long duration pulse transmission line 15 to the windings 50 to produce the pinch and subsequent D.C. B, field in place of the pinched field generator and crowbar arrangement.

In order to precisely describe the nature of the directcurrent magnetic field generated about the pinched plasma, it is desirable to describe the fields within the containment region 12 in some detail. The current. in the plasma 51 follows a confined circular path within the torus 41. For any relatively small segment along this path, the pinched plasma may be considered in the form of a column. The term encircling is used in the present discussion to connote that, as to this column, the plasma is encompassed by a unidirectional magnetic field which is concentric with the column in a plane transverse thereto. The application of a radio-frequency magnetic field which is orthogonal or crossed with respect to this directcurrent magnetic field is important for gaining stability in confinement of the plasma 51.

Immediately subsequent to the crowbarring accomplished by the short circuiting device 16, the power klystron tube 17 is turned on by the negative cathode pulse source 32. The pulse source is usually coupled (FIG. 2) to the switch control 55 to be energized at an appropriate instant. This action initiates the operation of the electron gun within the klystron tube 17 to provide the stream of bunched electrons as is normal to klystron operation.

The alternating fileds established within the resonant cavity 10 by the bunched electron flow are preferably of the TM mode and are represented as shown in the field distributions in the simplified diagram of FIG. 3. Within the resonant cavity 10 the radio-frequency magnetic fields extend concentrically about the central axis 18. The RF. electric field vectors are represented as vertically extending arrows, and the radio-frequency magnetic field as dotted lines which are normal to the directcurrent magnetic field lines. Taking a columnar portion of the circular current path, the radio-frequency magnetic fields lie, in this region-immediately adjacent the plasma, along lines parallel to the path of the plasma current. This may, for purposes of reference, be spoken of as the B direction. The radio-frequency magnetic fields are thus crossed with respect to the direct-current magnetic field which encircles the plasma current, and which may be spoken of as being in the B, direction.

A description of the manner in which dynamic stability is established through the use of combined crossed static and alternating magnetic fields is provided in an article entitled Radiation Pressure Confinement, the Shock Pinch and Feasibility of Fusion Propulsion, by Milton U. Clauser and Erich S. Weibel, as reporated on page 161 in the Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy, published by the United Nations in 1958. Reference may be made to that article for a description of the theoretical and practical considerations which make such a magnetic field relationship desirable. Briefly,

' however, it is shown that the combined use of radiofrequency B and direct-current B magnetic fields results, where B B,, in substantial reduction if not elimination of all of the principal instabilitim affecting a pinched plasma. The so-called sausage, kink, and flute instabilities are each prevented by this field configuration.

In accordance with the present invention, not only is v the need for separate coupling equipment to provide the desired radio-frequency magnetic field in the region of the plasma eliminated, but also the coupling arrangement is simplified by mounting-the linear windings 50 externally to the cavity 10. Furthermore, this external D.C. B drive, outside the resonant cavity 10, substantially improves the problem of R.F. electrical field breakdown. Also, the division of the resonant cavity into four or more radial sections, as by the grooves 40, will prevent shunting of the B, drive. Moreover, the resonant cavity 10 is easily divided into two or more sections so that it may be detached for assembly and maintenance, purposes.

I claim: 7

1. Apparatus for providing stable confinement of a gaseous plasma, the particles of which have high kinetic energizes, the apparatus including in combination: a hollow torus within which a gaseous plasma is maintained; a segmented resonant cavity microwave member secured to inner surfaces of said torus; a magnetic drive device including conductors coupled to the outer surface of said torus for establishing a high density current in the gaseous plasma, the current being directed along a selected path and generating an encircling magnetic field; and an electron discharge device providing a velocity modulated electron stream electromagnetically coupled and interacting with the segmented resonant cavity for providing radio-frequency magnetic fields therein which are crossed with respect to the encircling magnetic field about the current, the radio frequency magnetic fields being substantially greater than the encircling magnetic field.

2. A high kinetic energy gas confinement arrangement comprising: a hollow torus defining a toroidal gas plasma containment region; segmented conductive layers within said torus and energiza'ole by high frequency electric energy to provide radio-frequency electromagnetic fields within the torus; linear windings encompassing the outer surface of said torus; means for energizing said linear windings to develop sustained high current flow therethrough whereby the conductive. plasma within the torus will suffer a pinch effect; and means for energizing the segmented portions during substantially maximum current flow in said linear windings to stabilize the pinch effect in the conductive plasma, the magnetic field produced by said radio frequency energy'being substantially greater than the magnetic field produced by said linear windings.

3. Apparatus for providing stable confinement of a temperature gaseous plasma, comprising: a hollow torus within which the gaseous plasma is maintained; a segmented resonant cavity microwave member secured to inner surface of said torus; a plurality of linear windings surrounding said torus on the outer surface thereof and being concentric with the axis of revolution of said torus; a magnetic drive device coupled to said winding for inducing a high density current in the gaseous plasma, the current being directed along a selected path and generating an encircling magnetic field around the gaseous plasma; and means coupled to said torus and interacting with the segmented resonant cavity for providing radio-frequency magnetic fields therein which are crossed with respect to the encircling magnetic fields about the current, the radio frequency magnetic fields being substantially greater than the encircling magnetic field.

4. A high temperature gas confinement arrangement comprising: a hollow torus defining a toroidal gas containment region; segmented conductive layers on the inner surface of said torus and being energizable by high frequency electric energy to provide radio frequency electromagnetic fields within the torus; linear windings encompassing the outer surface of said torus, said windings being po'axial with the axis of revolution of said torus; means for energizing said linear windings to develop fast rising, sustained high current flow therethrough whereby the gas within the torus will suffer a pinch effect; and high power microwave generating means coupled to said torus for energizing the segmented portions during substantially maXimum'curren-t flow in said linear windings, the magnetic field produced by said radio frequency energy being substantially greater than the magnetic field produced by said linear windings.

References Cited in the file of this patent UNITED STATES PATENTS 3,022,236 Ulrich a a1. Feb. 20, 1962 3,029,361 Hernquist Apr. 10, 1962 3,037,921 Tuck June 5, 1962 FOREIGN PATENTS 824,765 Great Britain Dec. 2, 1959 

1. APPARATUS FOR PROVIDING STABLE CONFINEMENT OF A GASEOUS PLASMA, THE PARTICLES OF WHICH HAVE HIGH KINETIC ENERGIZES, THE APPARATUS INCLUDING IN COMBINATION: A HOLLOW TORUS WITHIN WHICH A GASEOUS PLASMA IS MAINTAINED; A SEGMENTED RESONANT CAVITY MICROWAVE MEMBER SECURED TO INNER SURFACES OF SAID TORUS; A MAGNETIC DRIVE DEVICE INCLUDING CONDUCTORS COUPLED TO THE OUTER SURFACE OF SAID TORUS FOR ESTABLISHING A HIGH DENSITY CURRENT IN THE GASEOUS PLASMA, THE CURRENT BEING DIRECTED ALONG AT SELECTED PATH AND GENERATING AN ENCIRCULATING MAGNETIC FIELD; AND AN 